ANGIOTENSIN-II INHIBITION OF L-TYPE CA2+ CURRENT IN SINOATRIAL NODE CELLS OF RABBITS

The actions of angiotensin II (ANG II) were examined in the spontaneously active cells isolated from the rabbit sinoatrial node, using the nystatin-permeabilized, whole cell, patch-clamp method. At 30 nM, ANG II significantly lowered the spontaneous firing rate of the action potentials from 212 +/- 21 to 172 +/- 32 beats/min, with a concomitant reduction in the action potential amplitude. The voltage-clamp experiments showed that ANG II inhibited the L-type Ca2+ current (I-Ca) with a dissociation constant (K-d) of similar to 4 nM and a maximal inhibition of 30%. The inhibition was blocked by an AT(1)-receptor antagonist CV11974. Acetylcholine (ACh) at 10 mu M reduced the I-Ca by 42 +/- 12%, and ANG II did not cause any further inhibition in the presence of ACh. At 100 nM, ANG II reduced the I-Ca by only 12% in the presence of 2 mu M isoproterenol, and a similar inhibition was observed with 0.1 mu M ACh. ANG II did not affect the dibutyryl adenosine 3',5'-cyclic monophosphate-stimulated I-Ca. Protein kinase C activator 12-O-tetra-decanoylphorbol-13-acetate did not mimic ANG II in the effects on I-Ca, and preincubation of the cells with calphostin C, a protein kinase C inhibitor, did not attenuate the ANG II effect. ANG II exerts a negative chronotropic effect in the pacemaker cells as its direct action through a pathway involving adenosine 3',5'-cyclic monophosphate-dependent protein kinase.